The present invention relates to a heat process for producing corrosion resistant steel articles, comprising a first step during which the articles are nitrided in a gaseous atmosphere to form on their surface a base layer consisting essentially of epsilon type iron nitride and a second step during which the articles are exposed to an oxidizing atmosphere to form an oxide-rich outermost layer over the base layer of the articles.
Nitriding and nitrocarburizing are processes designed to improve resistance to fatigue, wear, binding and friction. To date, ionic nitriding, certain salt bath processes and certain gaseous processes have proved satisfactory methods of producing nitrided layers having good tribological properties.
Recent attempts at improvements have focussed for the most part on the corrosion resistance of nitrided and nitrocarburized articles. Many such articles, including cooking moulds, glass-making moulds and mechanical parts, are used at temperatures above the ambient temperatures; others are used in fairly corrosive fluid environments (e.g., valves and fittings).
European Pat. Nos. 77,627 and 122,762 describe processes which consist in nitriding or nitrocarburizing steel articles, followed by oxidation in a gaseous atmosphere to provide the articles with a protective oxidized coating.
Oxidation is a discontinuous step which takes place after the nitriding step and requires a concentration of residual air and/or water vapour sufficient to oxidize the articles emerging from the furnace at a temperature of the order of 570.degree. C. This step normally lasts between 5 and 120 seconds. It should be noted that this process does not allow for an oxidation which can be properly controlled or reproduced. U.S. Pat. No. 4,131,492 describes a process whereby nitriding is followed by air oxidation to form an oxidized surface layer on the steel articles, the oxidized layer being such that it is more easily abraded than the nitrided layer, a property which allows for the production of mechanical joints and gears the components of which engage rapidly with little friction. This process does, however, have a drawback in that the oxidized layer has lower surface resistance to abrasion than the nitrided layer. This weak surface resistance to abrasion stems from poor adhesion of the oxidized layer to the nitrided layer.
French Pat. No. 2,560,892 describes a process for ionic nitriding of steel articles. The process consists in providing a gaseous plasma at very low pressure of only a few torr and then forming a nitrided layer on the article, followed by an oxidized surface layer.
This is a very particular method of nitriding. Owing to its characteristics, e.g., low pressure and placement of articles on a cathode, this process does not readily lend itself to industrial application and does not normally make use of ammonia as the nitriding atmosphere. Indeed, the plasma which is generated triggers dissociation of the nitrogen molecules into atomic nitrogen which procides the nitriding atmosphere. This process calls simply for introducing nitrogen into the process chamber to produce nitriding, as opposed to gaseous nitriding wherein ammonia is required to produce active nitrogen. The same phenomenon applies for the oxygen required for oxidation. Furthermore, the method described in this patent entails slow reaction kinetics, a characteristic which does not favour its practical application.
U.S. Pat. No. 2,343,418 describes a process whereby non-reflective aircraft wings are produced by first nitriding such parts to render them resistant to abrasion and then oxidizing them by introducing air into the treatment furnace.